When Cliff Swallows arrive on the breeding grounds in North America, the dirty work begins. The swallows scoop up mud in their beaks and carefully build a gourd-shaped nest with a tapered opening. They add a lining of dry grass to keep eggs warm. It takes days of work and a thousand mouthfuls of mud to finish a single nest—and it’s just one part of a large colony. Learn more at BirdNote.org.